Use of different process gases for manufacturing isolating alumina coatings by flame spraying with cords

Besides conventional industrial demands, thermally sprayed coatings are increasingly used for innovative products. Such an application is the additive manufacturing of electrical components in automotive engineering. In particular, heating units are currently manufactured by various spray technologies. At present, simpler spraying processes like flame spraying are investigated with regard to their suitability as a future cost-effective alternative for fabricating isolating alumina coatings. According to the literature, flame spraying using alumina cords resulted in homogeneous coatings with porosity, microstructure and coating formation comparable to plasma spraying, likewise revealing the presence of metastable phase. However, these investigations were carried out until now with alumina cords having relatively coarse powder fractions and using solely pressurized air as atomizing gas. Recently, a new quality of cords providing finer microstructures is commercially available for flame spraying. Moreover, alternative gases can be used to increase the particle velocity and thus change energy upon impact on the substrate. Thus, argon as well as pressurized air were used as atomizing gases to produce flame sprayed alumina coatings with a modern cord in different thicknesses and varying stand-off distances. The results demonstrate higher deposition efficiency, finer surface structures and a more regular and higher resistivity compared to past investigations.